1. Field of the Invention
The invention relates generally to pseudolites for satellite positioning systems and more particularly to a pseudolite for transmitting a positioning system signal having identification information corresponding to a positioning system satellite that is currently out-of-view.
2. Description of the Prior Art
The global positioning system (GPS) operated by the United States government uses microwave transmissions from orbiting satellites with known orbits. These transmissions are received by a satellite navigation receiver for determining the location of the receiver. Such microwave frequencies are blocked by the earth or by local obstructions such as buildings. Locating the receiver in an urban city block or inside a building can severely limit performance by excluding necessary satellites from a constellation being tracked.
Each of the GPS satellites transmits a GPS signal on the same carrier frequency modulated by location-determination information from that GPS satellite and spread by pseudorandom (PRN) codes that are distinct for that GPS satellite. Two different PRN codes are used by each satellite: a long code termed the precise/encrypted (P/Y) code and a short code of 1023 bits or chips termed the coarse/acquisition (C/A) code. Either the P/Y code or the C/A code identifies the GPS satellite transmitting the GPS signal and enables a GPS navigation receiver to distinguish the GPS signal from one GPS satellite from the GPS signal from another GPS satellite. The P/Y code is encrypted and restricted for use to those authorized by the United States Department of Defense while knowledge of the C/A code is available to all users.
There are over one thousand distinct C/A PRN codes that could be used for identification for distinguishing the GPS satellites. Of these C/A PRN codes, the United States government has currently allocated about thirty-two for use by GPS satellites. Existing GPS receivers are designed to search for GPS signals from GPS satellites having any one of these codes. Of these thirty-two allocated satellites, currently about twenty-seven are orbiting and operational and five are not operational. Typically, of the orbiting operational satellites, in mid-latitudes about one-third will be above the Earth horizon and potentially receivable by a GPS receiver and about two-thirds will be unreceivable to the receiver by being below the horizon. Therefore, about twenty-three of the satellites will not be receivable for use by the GPS receiver.
Existing GPS applications use pseudolites to augment the satellite constellation and thus improve availability of the GPS signal. Such pseudolites mimic the satellite transmissions by broadcasting pseudo GPS signals, but are fixed on the ground and transmit the location-determination information appropriate to the geographical location of the pseudolite. The pseudolites make use of PRN codes that have not been allocated for GPS satellites. Signal reception is nearly guaranteed when the pseudolite is located nearby due to relatively higher signal strength of the received pseudo GPS signal. In addition to the thirty-two PRN codes allocated for GPS satellites the United States government currently allocates about four codes for the use of pseudolites. For example, a pair of pseudolites at the end of an airport""s runway are conventionally used to enhance the position determination of a navigational receiver in a landing aircraft. It has been proposed that several pseudolites be used in a metropolitan area in order to improve GPS service in urban canyons and inside of buildings.
Fast GPS signal acquisition is important in applications for many GPS navigation receivers. For example, a battery powered receiver alternating between operational and standby modes needs a fast acquisition in order to have good battery life with small batteries. One technique for achieving a fast acquisition is to minimize the number of PRN codes or other types of identifications that are searched in order to acquire GPS signals. However, a requirement for more than four pseudolites increases the number PRN codes that must be stored or generated in a GPS receiver, thereby slowing signal acquisition time in certain circumstances.
There is a need for a pseudolite using a PRN code identification that minimizes the number of PRN codes that must be stored for search in a GPS receiver. Further, there is a need for an autonomous assignment procedure that simplifies and automates the choosing process, especially when there are a multiplicity of pseudolites to be installed, and the pseudolites could be supplied by different manufacturers.
It is therefore an object of the present invention to provide a positioning system (PS) pseudolite for a satellite positioning system where the PS pseudolite transmits a pseudo PS signal using a PS satellite identification for a PS satellite that is allocated for the positioning system but is currently unreceivable in the vicinity of the PS pseudolite.
The PS pseudolite of the present invention operates autonomously and independently from any central authority for determining which particular one of the unreceivable PS satellite identifications is used for transmitting a pseudo PS signal.
Briefly, in a preferred embodiment, a positioning system (PS) pseudolite of the present invention is a global positioning system (GPS) pseudolite. The GPS pseudolite mimics a global positioning system (GPS) signal by using a C/A pseudorandom (PRN) identification code for a GPS satellite that is allocated in the GPS system but is unreceivable in the local vicinity because it is below the local horizon in its orbit. The pseudolite transmits location-determination information including ephemeris data corresponding to the geographical location of the pseudolite. In order to ensure that two of the GPS pseudolites in the same vicinity do not use the identification PRN code from the same unreceivable GPS satellite, each GPS pseudolite listens first before transmitting to detect the identifications in the received GPS signals and uses PRN codes that are not being received. This autonomous code assignment process is independent of any central authority so that multiple vendors can supply pseudolites even in the same local area.
The GPS pseudolite includes a satellite availability calculator including a visibility calculator and an operational identifier. The visibility calculator uses GPS satellite almanac and/or ephemeris orbital parameter data for determining the GPS satellites that are in-view having a line-of-sight to the GPS pseudolite and the GPS satellites that are out-of-view, for example behind the Earth. The operational identifier determines the GPS satellites that are operational and those that are non-operational. Non-operational GPS satellites include those GPS satellites that have not been launched or have been turned off. The GPS satellites that are both operational and in-view are designated as receivable GPS satellites. The GPS satellites that are either out-of-view or non-operational are designated as unreceivable GPS satellites. The GPS pseudolite further includes a GPS receiver, a pseudolite detector, a satellite identification selector, and a pseudolite GPS transmitter. The GPS receiver receives GPS signals from the receivable GPS satellites and pseudo GPS signals from other GPS pseudolites in the vicinity and passes the received identifications to the pseudolite detector. The pseudolite detector detects that another GPS pseudolite in the vicinity is transmitting when a received identification matches the identification of an unreceivable GPS satellite. The satellite identification selector selects an available identification that both corresponds to one of the unreceivable GPS satellites and is not currently being used by another GPS pseudolite. The pseudolite transmitter then transmits a pseudo GPS signal having the selected available PRN code identification.
An advantage of the PS pseudolite of the present invention for a positioning system is that the PS pseudolite re-uses identifications that have been allocated for positioning system satellites, thus reducing the search time for a remote GPS receiver to acquire a pseudolite transmission.
Another advantage of the PS pseudolite of the present invention is that no coordination is needed by either manufacturers or system integrators with regard to selection of unique and independent identifications.
These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments which are illustrated in the various figures.